The present invention relates generally to forming patterns on or in a surface material, assemblies used therefor, and articles formed thereby. More specifically, the present invention relates to microscale patterning and/or lithography. Microscale patterning and microscale lithography have a broad spectrum of applications, e.g. in the production of integrated circuits, microdevices, and the like. The patterns formed can be utilized to perform an array of functions, including electrical, magnetic, optical, chemical and/or biological functions.
One of the key processing methods in fabrication of semiconductors, integrated electrical circuits, integrated optical, magnetic, and mechanical circuits and microdevices is forming very small patterns.
Lithography is often used to create a pattern in a thin film carried on a substrate so that, in subsequent process steps, the pattern will be replicated in the substrate or in another material which is added onto the substrate. One purpose the thin film satisfies is protecting a part of the substrate so that in subsequent replication steps, the unprotected portion can be selectively etched or patterned. Thus, the thin film is often referred to as a resist.
A typical lithography process for the integrated circuits fabrication involves exposing a resist with a beam of energetic particles which are electrons, or photons, or ions, by either passing a flood beam through a mask or scanning a focused beam. The particle beam changes the chemical structure of the exposed area of the film, so that when immersed in a developer, either the exposed area or the unexposed area of the resist will be removed to recreate the pattern or obverse of the pattern, of the mask. A limitation this type of lithography is that the resolution of the image being formed is limited by the wavelength of the particles, the particle scattering in the resist, the substrate, and the properties of the resist. Although pattern sizes greater than 200 nm can be achieved by photolithography, and pattern sizes in the range of 30 nm to 200 nm can be achieved utilizing electron beam lithography, these methods are resource intensity and suffer from low resolution.
U.S. Pat. No. 5,772,905 describes a method and apparatus for performing ultra-fine line lithography wherein a layer of thin film is deposited upon a surface of a substrate and a mold having at least one protruding feature and a recess is pressed into the thin film.
An alternative strategy to those described above is to use a xe2x80x9cnaturally occurringxe2x80x9d or xe2x80x9cself-assemblyxe2x80x9d structure as a template for subsequent parallel fabrication. For example, U.S. Pat. No. 4,407,695 and U.S. Pat. No. 4,801,476 describe a spin coating technique to prepare close-packed monolayers or colloidal polystyrene spheres with diameters of typically 0.1-10 microns on solid substrates. The pattern is then replicated by a variety of techniques, including evaporation through the interstices, ion milling of the spheres and/or the substrates, and related techniques. Highly ordered biologically membranes (xe2x80x9cS-layersxe2x80x9d) have also been suggested as starting points for fabrication. Close packed bundles of cylindrical glass fibers, which could be repeatedly drawn and repacked to reduce the diameters and lattice constant have also been used. Block copolymer films have been suggested for use as lithography masks wherein micelles of the copolymer which form on the surface of a water bath are subsequently picked up on a substrate.
To date, the focus of xe2x80x9cself-assemblyxe2x80x9d has been primarily on either phase separation of a polymer blend, of di-block copolymers, or of local modification of surface chemistry (i.e., chemical lithography). In self-assembly by phase separation, the periodic structures are multidomain, and their orientation and locations are uncontrollable and random. A long-sought after goal in self-assembly is precise control of the orientation and location of a self-assembled polymer structure.
There is an ongoing need to produce progressively smaller pattern size. There also exist a need to develop low-cost technologies for mass-producing microscale and sub-micron (e.g. nanometer) structures. Microscale, indeed nanoscale and smaller, pattern technology will have an enormous impact in many areas of engineering and science. Both the future of semiconductor integrated circuits and the commercialization of many innovative electrical, optical, magnetic, and mechanical microdevices that are far superior to current devices will depend on such technology.
Technologically, self-assembly promises not only low-cost and high-throughput, but also other advantages in patterning microstructures, which may be unavailable in conventional lithography.
The present invention is generally directed to the formation of patterns in a material through deformation induced by a mask placed above a material, as well as assemblies used therefor, and products formed thereby. An important aspect of the present invention is novel method, referred to herein as xe2x80x9clithographically-induced self-assemblyxe2x80x9d (LISA). In this process a mask is used to induce and control self-assembly of a deformable surface, preferably a thin film into a pre-determined pattern. One advantage of the present invention is relatively accurate control of the lateral location and orientation of a self-assembled structure. Preferably, a substantially uniform, film is cast on a substrate. A mask, preferably with protruding patterns representing the pattern to be formed in or on the film, is placed above the film, but physically separated from the film by a gap. The mask, the film, and the substrate are manipulated, if necessary, to render the film deformable. For example in the case of a polymer, the polymer film may be heated to a temperature above the polymer""s glass transition temperature and then cooled down to room temperature. During the heat-cool cycle, the initially flat film assembles into discrete periodic pillar arrays. The pillars, formed by rising against the gravitational force and surface tension, bridge the two plates to form periodic pillar arrays. The pillars generally have a height equal to the plate-mask separation. Moreover, if the surface of the mask has a protruding pattern, the pillar array is generally formed only under the protruding pattern with the edge of the array generally aligned to the boundary of the mask pattern. After the pillar formation, due to a constant polymer volume, there is little polymer left in the area between pillars. The shape and size of the mask pattern can be used to determine the pillar array""s lattice structure. The location of each pillar can be controlled by the patterns on the mask. This process can be used repeatedly to demagnify the self-assembled pattern size. This demagnification permits a self-assembled structure to have a size smaller than that of the mask pattern(s). If the demagnification is used repeatedly, a size much smaller than that by a single self-assembly process can be achieved. This would allow for progressively smaller pattern-mask-patterns to be formed. The basic LISA process can also be modified to form a non-pillared pattern that is substantially identical to the features of the mask.
One embodiment of the present invention is a patterning method or method of patterning which comprises depositing a material on a substrate. The material and substrate may be already formed, and the material and substrate may be the same or different. In this case the step of depositing a material would not be necessary, but rather a surface layer(s) would be selectively manipulated so that a pre-determined thickness of surface material is deformable. This thickness must be small enough that the mask can interact with the material through the separation distance to form a contact therebetween. As is described more fully herein, the thin film or surface layer(s) preferably has a thickness in the range of about 1 nm to about 2,000 nm, more preferably about 10 nm to about 1,000 nm, more preferably about 100 nm to about 500 nm and even more preferably about 50 nm to about 250 nm. If the deposited material is deformable at room temperature (e.g., a liquid polymer or polymer dispersion, the material may not need to be deformed). If a liquid polymer is used, it may be cured (e.g., photo curing) after either pillar formation, usually before removal of the mask. For a solid material, it may be necessary to render the material deformable, e.g. by heating to a temperature where the material may flow. Deforming by heat is a preferred route, but the material or surface layer(s) may also be deformed by other routes (e.g., chemical reactions). Heating may occur by any conventional means (e.g., laser, light sources, heat radiating or microwave induction), and the heat may be pulsed or continuous.
It is important that the mask be maintained above the material or film. A spacer (which may be integrally or non-integrally formed with the mask) is convenient to this end. However, an assembly may be used wherein the mask is maintained above said material without resorting to a spacer. The substrate can be any number of compositions which are capable of supporting the film, but the present invention has particular applicability to substrates which are, themselves intended to be processed to have patterns formed thereon or therein. The substrate can have pre-existing relief patterns or be flat.
The mask can be of any suitable material as described herein in many cases, the mask, will often be very similar in composition to the underlying substrate. Indeed, it is envisioned to use a suitably patterned substrate from a previous LISA process in a second or more LISA process or LISC process. The mask can have any suitable surface coating and the protrusion may be formed from a surfactant or other suitable protruding material (e.g., monolayers or self-assembled monolayers) with a different surface energy. The protruding pattern may be of varying heights on the same pattern resulting in like pillars. Of course, any combination of protrusion pattern protrusion coating or monolayer material pattern may be used to form the relief structure.
Another embodiment of the present invention is the relief structure formed by either or both the LISA and LISC process.